I have a map that is a bit of a labyrinth, filled with promoted enemies when most of your units are unpromoted. You have to use lopsided green units to your advantage, using armor knights to block the way for your mages to kill the enemy general. Same goes for fliers with archers. The map is supposed to show how your army can take on nearly impossible odds, proving that you can indeed win the war

I think it could be this.It's more than one map, but it's a single level, it's a castle that you have to explore to solve some puzzles until you reach the end.You can move through the rooms in the order you want, and when you return to the previous ones your progress is saved if you already completed it.

Way back in 2013 or so, before my old laptop was stolen and I lost about half of my projects, I was trying to make a Fire Emblem equivalent to Kaizo Mario, and a few chapters in there was a chapter called The Factory; the idea being that bandits are such a generic enemy, they were literally being churned out by a factory. At the top of the map, there were several of those cave/doorway tiles used in the ruins tileset, and every generic bandit boss from FE7 was dotted about here and there. They were the only enemy units, but every time one was defeated, they would immediately re-load out of one of the doorways and run back to their original location. This made for some "fun" situations where one of your units would kill a boss in retaliation and then the boss would immediately re-load and happen to end up next to another, weaker one of your units. The only way to win was to seize a throne, which was imagined to be the switch that turns off the factory, but if you just weakened the bosses instead of defeating them they would move to the throne to recover their HP. I hope to re-visit this concept someday in the form of a druid who has a shrine that empowers him to summon phantoms and the only way to stop the phantoms pouring out is to destroy the shrine.

edit: fwiw this was before Phoenix Mode existed; if Fates occupied a permanent position in my memory banks I wouldn't've posted this